The present invention relates generally to an anti-lock braking system for an automotive vehicle, and more specifically, to a method and apparatus for controlling the braking torque of the wheel in accordance with a predicted value of slip.
Anti-lock braking systems are commonly used in automotive vehicles to prevent the wheels from locking when the vehicle is over-braked. By preventing the wheels from locking the directional stability and steerability of the vehicle may be maintained. Each of the wheels is typically monitored separately and controlled separately. Each wheel has a wheel-speed sensor that monitors the rotational motion of the wheel. If one of the wheels shows signs of locking there is a sharp rise in the peripheral wheel deceleration and in wheel slip. If the wheel slip exceeds a defined value, a brake controller commands a solenoid valve unit to stop or reduce the build up of brake pressure. The brake pressure is subsequently increased to prevent an under-brake situation.
Typically, such systems merely monitor the slip rate or the wheel speed in determining whether to apply brake pressure or reduce brake pressure. The amount of reduction or increase in the application of brake pressure is typically a constant or an open loop value. The amount of pressure or torque is not typically taken into consideration. That is, a fixed amount of brake pressure is applied or removed. Likewise, only measured conditions are taken into account. That is the applied brake pressure is applied to the braking system according to past sensed conditions.
It would therefore be desirable to adjust an amount of braking torque or pressure to the vehicle wheels in response to sensed operating conditions and predicted future conditions of the vehicle rather than merely a fixed amount based upon wheel slip.
The present invention uses sensed and predicted vehicle conditions such as wheel slip to determine a braking torque for each wheel of the vehicle.
In one aspect of the invention, a control system for an automotive vehicle includes a wheel speed sensor generating a rotational speed signal and a controller coupled to the wheel speed sensor. The controller estimates a vehicle speed, calculates wheel slip based upon the vehicle speed and the rotational speed, calculates a predicted future wheel slip based upon the vehicle speed and the rotational speed, estimates a normal force on the wheel, calculates a modified brake torque signal in response to the wheel slip, the predicted future wheel slip and the normal force, and actuates the wheel brake in response to the modified brake torque signal.
In a further aspect of the invention, a method of controlling a vehicle having a wheel and wheel brake comprises measuring rotational speed of a wheel, determining a vehicle speed, calculating wheel slip based upon the vehicle speed and the rotational speed, calculating a future predicted wheel slip based upon the vehicle speed and the rotational speed, estimating a normal force on the wheel, calculating a modified brake torque signal in response to the wheel slip, the predicted future wheel slip and the normal force, and actuating the wheel brake in response to the modified brake torque signal.
One advantage of the invention is that an amount of braking torque to be applied for each vehicle is calculated using the varying conditions of the vehicle and predicted future conditions and thus a more accurate representation of the amount of brake torque to be applied may be determined. Consequently, the response of the anti-lock brake system is more rapid than previously known brake systems.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.